Fatty acid synthetase genes which are responsible for novel fatty acid synthesis have been cloned in a variety of organisms and thoroughly studied (e.g., Non-Patent Document 1: E. Schweizer et al., Microbiol. Mol. Biol. Rev., 68, 501-517 (2004)).
In some bacteria and in fungi and animals, so-called “type I” multifunctional enzymes such as those shown below are known to catalyze a series of reactions in fatty acid synthesis.
Type Ia: (Fungi) AC-ER-DH-MPT/ACP-KR-KR-KS-PPT, α6β6 (β+α: approx. 3,950 amino acids). (Bacteria) AC-ER-DH-MPT-ACP-KR-KS-PPT, α6, structure in which the β and α subunits of fungal fatty acid synthetase (FAS) are connected head-to-tail (α: approx. 3,000 amino acids).
Type Ib: (Animals) KS-AT-DH-ER-KR-ACP-TE, α2 (α: approx. 2,500 amino acids). The above abbreviations stand for the following:
AC: ac(et)yltransferase
AT: malonyl/acetyl-transferase
MPT: malonyl/palmitoyl-transferase
KS: ketoacyl synthase
KR: ketoacyl reductase
DH: dehydratase
ER: enoyl reductase
ACP: acyl carrier protein
TE: thioesterase
PPT: palmitoyl/palmitoyl-transferase.
In the yeast Saccharomyces cerevisiae (also abbreviated below as “S. cerevisiae”), novel fatty acid synthesis is carried out by a fatty acid synthetase (an α6β6 complex composed of β subunits encoded by a FAS1 gene and α subunits encoded by a FAS2 gene) up to 18 carbons (stearic acid). In addition, ELO1, ELO2 and ELO3 are known as fatty acid elongase genes. It is thought that ELO1 is responsible for extending the length of C12 to C16 chains to from C16 to C18, ELO2 is responsible for extending the length of C16 to C18 chains to C22, and ELO3 is responsible for extending the length of C18 to C24 chains to C26.
At the same time, the fact that, in the lipid-producing fungus Mortierella alpina (also abbreviated below as “M. alpina”), a mutant strain having a lowered fatty acid elongation activity from palmitic acid to stearic acid can be obtained by mutagenic treatment (Patent Document 1: Japanese Patent Application Laid-open No. 2001-245687) suggests that, at the very least, different enzymes are responsible for synthesis up to palmitic acid and for synthesis from palmitic acid to stearic acid.
However, the fatty acid synthetase genes which are responsible for novel fatty acid synthesis in lipid-producing fungi such as M. alpina have not previously been cloned.